Destruction of organic contaminants, particularly volatile organic contaminants (VOCs) when they are located in low permeability soils is a complex problem with limited cost-effective alternatives to reduce their impacts on health and the environment. Current practices for remediation in low permeability soils focus on excavation and disposal in landfills, heating of the soils beyond the boiling point of water and then capturing the vapour for treatment or sequestration, or large-scale mixing of the soils with cements or other solidifying agents mixed with treatment compounds to degrade the VOCs. These current practices involve transplacement of the contaminants from one location to another, leave the contamination in place, or have very environmentally unfriendly carbon footprints.
The use of heat applied to the subsurface to volatilize contaminants and their subsequent capture via an applied vacuum is a commercially available technology. Heat is either delivered through thermal conductive heating (TCH), where vertical heater wells are heated to high temperatures and the heat is allowed to migrate through the soil, increasing the temperature of the soil as it conducts, or through electrical resistance heating (ERH) where an alternating current is applied to the soil via emplaced electrodes, and the resistance to the conduction of electricity results in heating of the soil. ERH is the subject of U.S. Pat. Nos. 7,290,459 and 5,656,239 and TCH is the subject of U.S. Pat. No. 5,114,497.
The use of oxidants, specifically persulfate, to degrade contaminants in soil and groundwater is a commercially available approach and is the subject of U.S. Pat. No. 6,019,548. Oxidation approaches are historically limited to soils where the oxidant can be effectively injected in a liquid mixture, limiting their use to permeable soils such as sands. For the particular oxidant persulfate, an additional activation step is required to produce the effective oxidizing radical. Activation processes include mixing with a metal ion, base activation where the pH of the system is raised well above natural groundwater levels, peroxide activation, or heating the mixture to temperatures above natural groundwater systems, but not to boiling temperatures, or some combination of the four.
Recent advances in the field of contaminant hydrogeology have shown that electrokinetics can be used to deliver remediation agents (through processes called electromigration or electroosmosis) such as oxidants, reductants, or electron donors to contaminants located in low permeability soils. This approach is commercially available and is the subject of U.S. Pat. No. 7,547,160. Historical use of electrokinetics in the remediation of soil and groundwater has focused on the use of electroosmosis to migrate contaminants to treatment zones, as opposed to the migration of treatment fluids to the contaminants. The use of electroosmosis in the remediation of soils and groundwater is commercially available and is the subject of numerous U.S. patents (U.S. Pat. Nos. 5,398,756; 5,476,992; 5,584,980; 5,725,752).
The combination of direct current electromigration and/or electroosmosis of persulfate oxidant in low permeability soils, and alternating current heating of the soils to provide activation of the persulfate oxidant by heat overcomes many of the limitations or environmental impacts of existing approaches. The advantage of the new process is that contaminants are treated in place with moderate application of external energy, and a much reduced carbon footprint than conventional heat-based approaches.